Silica Raman scattering probe for absolute calibration of Thomson scattering spectrometers

Abstract

A: We have developed a solid state probe for an absolute irradiance calibration of the Thomson scattering system on the Large Plasma Device (LAPD), based on Raman scattering off silica. Measurements performed with a triple-grating spectrometer have investigated the intensities of a pulsed laser beam Raman scattered off crystalline and amorphous silica over a range of temperatures of relevance to the LAPD (299-498 K). The data were compared with Rayleigh and Raman scattering intensities in gaseous nitrogen. The measurements show that Raman scattering off quartz allows rapid and accurate alignment and calibration of Thomson scattering systems in plasma physics experiments that cannot be calibrated using conventional methods. K: Plasma diagnostics -charged-particle spectroscopy; Plasma diagnostics -probes; Plasma generation (laser-produced, RF, x ray-produced)

Thomson scattering on the Large Plasma Device

“…This factor agrees within 10% with a redundant calibration obtained from Raman scattering off of a quartz crystal, that can be inserted into the scattering volume in lieu of the gas 15 .…”

“…The fine structure (blue) is mostly washed out by the instrument function, which allows only to resolve the brightest lines. A theoretical fit for a gas temperature of 298 K obtained as described in detail elsewhere 4 and convoluted with the instrument function reproduces the measured spectrum well (orange), including the difference in brightness of the red-shifted Stokes and the blue-shifted anti-Stokes lines 15 . Although the notch blocks most of the elastic Rayleigh scattering line that has an amplitude about 2,000 times that of the Raman signal, a small fraction leaks through the edge of the notch and contributes to the measured spectrum close to λ i .…”

“…The spectrum consists of dozens of peaks on either side of λ i . Each line corresponds to a different rovibrational transition from one rotational state to another, induced by the inelastic scattering process 15 . The fine structure (blue) is mostly washed out by the instrument function, which allows only to resolve the brightest lines.…”

First Results from the Thomson Scattering Diagnostic on the Large Plasma Device

“…A notch mask is placed in between the double-subtractive spectrometers to remove light at λ i . A 0.75 mm wide and 50 µm thick stainless steel mask blocks a wavelength range of 1.5 nm with an extinction ratio of 2.0 ×10 4 integrated over the entire spectrum [30,31]. The notch mask also acts to reduce the intensity of the broad Lorentzian wings of the instrument-broadened laser line outside of the 1.5 nm blocking width, since the 532 nm signal is removed before light reaches and scatters off of the second and third gratings (DG2 and DG3).…”

“…where k is the experimental throughput parameter that is exactly the same for all types of scattering, and can be determined in situ using Rayleigh [34] or Raman [35] scattering off of a known gas or solid target [30]. Maximizing the photon collection efficiency requires maximizing k.…”